For various reasons it is desirable to cool the material of gas turbine engine exhaust nozzles. Pressurized air from the gas turbine compressor can be used. This has several disadvantages. The air is heated by compression and therefore operates at an elevated temperature. Significant energy is consumed by compressing the air thereby decreasing the overall efficiency and power output of the engine. While such excess consumption of power is tolerable during the after burning mode, it is undesirable during long term operating conditions.
In order to reduce such cooling air flow requirements in advanced exhaust nozzles, external sources of cooling air such as blowers or ejectors are being considered. However, wall pressures on divergent flaps and sidewalls of conventional nozzles are too high to use these low pressure cooling sources. One solution to this problem is to convert the nozzle to a plug configuration. Existing 2-D plug test data show that wall pressures are almost always equal to ambient pressure along plug surfaces downstream of the throat plane.
The plug configuration however, is not suitable for the after burning mode of operation because its large surface area requires an excessive amount of cooling air; accordingly it is desirable that the nozzle be of the usual convergent/divergent form for after burning operation.